@946898963 2017-07-31T11:19:37.000000Z 字数 5893 阅读 1341

线程未捕获异常的处理

Java多线程

线程未捕获异常处理

线程中抛出的异常并不能被catch到，如下：

try{
    new Thread(new Runnable() {
        @Override
        public void run() {
            int i=1/0;
        }
    }).start();
}catch (Exception e){
    System.out.println("error:"+e.getMessage());
}

Thread中的运行时异常并不能被catch捕获，System.out.println("error:"+e.getMessage())并没有执行。
如果我们想要对Thread中抛出的异常进行处理，我们可以给线程设置异常捕获处理器。

Thread thread =new Thread(new Runnable() {
     @Override
     public void run() {
        int i=1/0;
     }
});
thread.setUncaughtExceptionHandler(new Thread.UncaughtExceptionHandler() {
    @Override
    public void uncaughtException(Thread t, Throwable e) {
        System.out.println("error:"+e.getMessage());
    }
 });
thread.start();

线程中抛出的异常会被setUncaughtExceptionHandler方法设置的异常捕获处理器捕获， System.out.println("error:"+e.getMessage())获得执行。

我们可以给每个线程的设置具体的未捕获异常处理器，也可以给所有的线程设置默认异常处理器。通过

Thread.setDefaultUncaughtExceptionHandler(new Thread.UncaughtExceptionHandler() {
    @Override
    public void uncaughtException(Thread t, Throwable e) {
    }
});

可以给所有线程设置默认的处理器。给所有的线程设置了默认的处理器后，对于没有设置特定的处理器的线程，当线程中出现未捕获的异常的时候，会使用默认的处理器来处理线程中出现的未捕获的异常。

如果没有给线程设置特定的处理器，那么线程的处理器就是线程的ThreadGroup对象，我们通过Thread.setDefaultUncaughtExceptionHandler设置的异常处理器的uncaughtException方法会在ThreadGroup对象的uncaughtException方法中被调用，具体看下一小节源码分析。

未捕获异常处理流程代码分析

当线程中出现未捕获异常后JVM将调用Thread中的dispatchUncaughtException方法把异常传递给线程的未捕获异常处理器。

/**
* Dispatch an uncaught exception to the handler. This method is
* intended to be called only by the JVM.
*/
private void dispatchUncaughtException(Throwable e) {
    getUncaughtExceptionHandler().uncaughtException(this, e);
}

dispatchUncaughtException这个方法只是提供给JVM调用的。

 /* Returns the handler invoked when this thread abruptly terminates
     * due to an uncaught exception. If this thread has not had an
     * uncaught exception handler explicitly set then this thread's
     * <tt>ThreadGroup</tt> object is returned, unless this thread
     * has terminated, in which case <tt>null</tt> is returned.
     * @since 1.5
     * @return the uncaught exception handler for this thread
     */
    public UncaughtExceptionHandler getUncaughtExceptionHandler() {
        return uncaughtExceptionHandler != null ?uncaughtExceptionHandler : group;
    }

从代码可以看出如果没有设置uncaughtExceptionHandler，将使用线程所在的线程组来处理这个未捕获异常。线程组ThreadGroup实现了UncaughtExceptionHandler，所以可以用来处理未捕获异常。ThreadGroup类定义：

class ThreadGroup implements Thread.UncaughtExceptionHandler

ThreadGroup实现的uncaughtException如下：

    /**
     * Called by the Java Virtual Machine when a thread in this
     * thread group stops because of an uncaught exception, and the thread
     * does not have a specific {@link Thread.UncaughtExceptionHandler}
     * installed.
     * <p>
     * The <code>uncaughtException</code> method of
     * <code>ThreadGroup</code> does the following:
     * <ul>
     * <li>If this thread group has a parent thread group, the
     *     <code>uncaughtException</code> method of that parent is called
     *     with the same two arguments.
     * <li>Otherwise, this method checks to see if there is a
     *     {@linkplain Thread#getDefaultUncaughtExceptionHandler default
     *     uncaught exception handler} installed, and if so, its
     *     <code>uncaughtException</code> method is called with the same
     *     two arguments.
     * <li>Otherwise, this method determines if the <code>Throwable</code>
     *     argument is an instance of {@link ThreadDeath}. If so, nothing
     *     special is done. Otherwise, a message containing the
     *     thread's name, as returned from the thread's {@link
     *     Thread#getName getName} method, and a stack backtrace,
     *     using the <code>Throwable</code>'s {@link
     *     Throwable#printStackTrace printStackTrace} method, is
     *     printed to the {@linkplain System#err standard error stream}.
     * </ul>
     * <p>
     * Applications can override this method in subclasses of
     * <code>ThreadGroup</code> to provide alternative handling of
     * uncaught exceptions.
     *
     * @param   t   the thread that is about to exit.
     * @param   e   the uncaught exception.
     * @since   JDK1.0
     */
    public void uncaughtException(Thread t, Throwable e) {
        if (parent != null) {
            parent.uncaughtException(t, e);
        } else {
            Thread.UncaughtExceptionHandler ueh =
                Thread.getDefaultUncaughtExceptionHandler();
            if (ueh != null) {
                ueh.uncaughtException(t, e);
            } else if (!(e instanceof ThreadDeath)) {
                System.err.print("Exception in thread \""
                                 + t.getName() + "\" ");
                e.printStackTrace(System.err);
            }
        }
    }

线程组处理未捕获异常，首先将异常消息通知给父线程组；如果父线程组为空，尝试利用一个默认的defaultUncaughtExceptionHandler获得异常处理器来处理异常，Thread.getDefaultUncaughtExceptionHandler()获得的异常捕获处理器就是我们之前通过Thread.getDefaultUncaughtExceptionHandler(）设置的处理器；如果没有默认的异常处理器则将错误信息输出到System.err。

如果不给线程设置线程组，仍然可以正常的使用默认异常处理器。所有的线程都会有线程组，即使我们在构造线程时显示的传入null值的线程组，最终JVM也会为我们分配到一个线程组。Thread的init方法决定了这个特性：

    if (g == null) {
            /* Determine if it's an applet or not */
            /* If there is a security manager, ask the security manager
               what to do. */
            if (security != null) {
                g = security.getThreadGroup();
            }
            /* If the security doesn't have a strong opinion of the matter
               use the parent thread group. */
            if (g == null) {
                g = parent.getThreadGroup();
            }
    }

几个小例子

    Thread.setDefaultUncaughtExceptionHandler(new Thread.UncaughtExceptionHandler() {
        @Override
        public void uncaughtException(Thread t, Throwable e) {
                System.out.println("I catch a exception from  " + Thread.currentThread().getName() + ":" + Thread.currentThread().getThreadGroup().getName());
            }
        });
        ThreadGroup myGroup = new ThredGroup("ThreadGroup");
        new Thread(myGroup, new Runnable() {
            @Override
            public void run() {
                int i = 1/0;
            }
        }, "thread1").start();
        new Thread(myGroup, new Runnable() {
            @Override
            public void run() {
                int i = 1/0;
            }
        }, "thread2").start();

打印结果如下：

I catch a exception from  thread1:myGroup
I catch a exception from  thread2:myGroup

通过自定义ThreadGrop改变上述机制。

    class OtherGroup extends ThreadGroup{
        public OtherGroup(String name) {
            super(name);
        }
        @Override
        public void uncaughtException(Thread t, Throwable e) {
            System.out.println("I am a other group and do nothing");
        }
    }
     Thread.setDefaultUncaughtExceptionHandler(new Thread.UncaughtExceptionHandler() {
        @Override
        public void uncaughtException(Thread t, Throwable e) {
                System.out.println("I catch a exception from  " +  Thread.currentThread().getName() + ":" + Thread.currentThread().getThreadGroup().getName());
            }
        });
        ThreadGroup myGroup = new OtherGroup("OtherGroup");
        new Thread(myGroup, new Runnable() {
            @Override
            public void run() {
                int i = 1/0;
            }
        }, "thread1").start();
        new Thread(myGroup, new Runnable() {
            @Override
            public void run() {
                int i = 1/0;
            }
        }, "thread2").start();

打印结果如下：

I am a other group and do nothing
I am a other group and do nothing

如果在OtherGroup的uncaughtException加上如下代码：

super.uncaughtException(t,e);

那么执行结果会是：

I catch a exception from  thread1:badGroup
I am a bad group and do nothing
I catch a exception from  thread2:badGroup
I am a bad group and do nothing

参考文献：
Java核心技术讲义
Java线程未捕获异常处理
 《Java并发》：第五节线程异常处理
 Java多线程——<七>多线程的异常捕捉

线程未捕获异常的处理

线程未捕获异常处理

未捕获异常处理流程代码分析

几个小例子

内容目录